The present invention relates to a process for polymerizing monovinylidene aromatic monomers, such as styrene, to produce polymers having a high degree of syndiotacticity, using a catalyst composition comprising a Group 4 metal complex.
In U.S. Pat. No. 4,680,353, there is disclosed a process for the preparation of syndiotactic polymers of monovinylidene aromatic monomers, using a catalyst system comprising a titanium catalyst and an alumoxane cocatalyst. However, this process uses relatively high amounts of cocatalyst which increases the cost of production.
WO93/03067 discloses a process for the preparation of syndiotactic polymers using a catalyst comprising a borane metal complex. However, this process generally yields lower catalyst activity and higher residual metal in the polymer.
Therefore, there remains a need for an activating cocatalyst composition which yields high catalyst activity and requires less aluminoxane, thus producing a polymer having lower levels of residual aluminum.
One aspect of the present invention is an activating cocatalyst composition used in the production of syndiotactic polymers from monovinylidene aromatic monomers wherein the composition comprises an aluminoxane and an electrophilic borane compound.
Another aspect of the present invention is a process for preparing syndiotactic polymers from monovinylidene aromatic monomers comprising contacting at least one polymerizable monovinylidene aromatic monomer under polymerization conditions with a catalyst composition comprising:
a) a Group 4 metal complex; and
b) an activating cocatalyst composition comprising an aluminoxane and an electrophilic borane compound.
This activating cocatalyst composition and process allows for the reduction in the amount of aluminoxane needed for high catalyst activity, thus decreasing the amount of residual aluminum in the polymer and lowering catalyst cost. The resulting syndiotactic polymers may be used in the preparation of articles such as a moldings, films, sheets and foamed objects.
The present invention is a method of producing a syndiotactic monovinylidene aromatic polymer. As used herein, the term xe2x80x9csyndiotacticxe2x80x9d refers to polymers having a stereoregular structure of greater than 50 percent syndiotactic of a racemic triad as determined by 13C nuclear magnetic resonance spectroscopy. Such polymers may be usefully employed in the preparation of articles and objects (for example, via compression molding, injection molding or other suitable technique) having an extremely high resistance to deformation due to the effects of temperature.
In the practice of the present invention, suitable monovinylidene aromatic monomers useful in preparing the syndiotactic monovinylidene aromatic polymers include those represented by the formula: 
wherein each R* is independently hydrogen; an aliphatic, cycloaliphatic or aromatic hydrocarbon group having from 1 to 10, more suitably from 1 to 6, most suitably from 1 to 4, carbon atoms; or a halogen atom. Examples of such monomers include, styrene, chlorostyrene, n-butylstyrene, vinyltoluene, and xcex1-methylstyrene, with styrene being especially suitable. Copolymers of styrene and the above monovinylidene aromatic monomers other than styrene can also be prepared.
The catalyst composition used in the process of the present invention comprises a Group 4 metal complex and an activating cocatalyst composition. All reference to the Periodic Table of the Elements herein shall refer to the Periodic Table of the Elements, published and copyrighted by CRC Press, Inc., 1989. Also, any reference to a Group or Series shall be to the Group or Series as reflected in this Periodic Table of the Elements, utilizing the IUPAC system for numbering groups.
The Group 4 metal complex preferably corresponds to the formula:
CpmMXnXxe2x80x2p
wherein:
Cp is a single xcex75-cyclopentadienyl or xcex75-substituted cyclopentadienyl group, the substituted cyclopentadienyl group being optionally also bonded to M through a substituent X;
M is a metal of Group 4 or the Lanthanide Series of the Periodic Table;
X each occurrence is an inert anionic ligand of up to 20 nonhydrogen atoms and optionally X and Cp are joined together;
Xxe2x80x2 is an inert, neutral donor ligand;
m and p are independently 0 or 1;
n is an integer greater than or equal to 1; and
the sum of m and n is equal to the oxidation state of the metal.
Illustrative but nonlimiting examples of X include hydrocarbyl, silyl, halo, NR2, PR2, OR, SR, and BR2, wherein R is C1-20 hydrocarbyl.
Illustrative but nonlimiting examples of Xxe2x80x2 include ROR, RSR, NR3, PR3, and C2-20 olefins or diolefins, wherein R is as previously defined. Such donor ligands are able to form shared electron bonds but not a formal covalent bond with the metal.
Preferred monocyclopentadienyl and substituted monocyclopentadienyl groups for use according to the present invention are more specifically depicted by the formula: 
wherein:
M is titanium;
X independently each occurrence is hydrogen, halide, R, or OR;
R is C1-10 hydrocarbyl group;
Xxe2x80x2 is a C4-40 conjugated diene;
n is 1, 2 or 3;
p is 1 when n is 1, and p is 0 when n is 2 or 3;
Rxe2x80x2 is in each occurrence independently selected from the group consisting of hydrogen, halogen, R, NR2, PR2; OR; SR or BR2, or one or two pairs of adjacent Rxe2x80x2 hydrocarbyl groups are joined together forming a fused ring system.
Preferably, the cyclic moiety comprises a cyclopentadienyl- indenyl-, fluorenyl-, tetrahydrofluorenyl-, or octahydrofluorenyl-group or a C1-6 hydrocarbyl substituted derivative thereof, n is three, p is zero, X is C1-4 alkyl or alkoxide. Most highly preferred metal complexes comprise pentamethylcyclopentadienyltitanium trimethyl, pentamethylcyclopentadienyltitanium tribenzyl, pentamethylcyclopenta-dienyltitanium trimethoxide, octahydrofluorenyltitanium tribenzyl, octahydrofluorenyltitanium trimethyl or octahydrofluorenyltitanium trimethoxide.
In a preferred embodiment, the metal complex is a metal trialkoxide which is combined with a trialkylaluminum or trialkylboron compound such as triethyl aluminum, tri-n-propyl aluminum, tri-isopropyl aluminum, tri-n-butyl aluminum, triisobutyl aluminum, and mixtures thereof, either prior to or simultaneously with the activating cocatalyst composition to form the active catalyst composition. It is believed, without wishing to be bound by such belief that the trialkylaluminum compound or trialkylboron compound causes the in situ transfer of the alkyl group to the Group 4 metal complex prior to activation thereof.
The Group 4 metal complexes are rendered catalytically active by combination with an activating cocatalyst. The cocatalyst composition of the present invention comprises an aluminoxane and an electrophilic borane compound.
Suitable aluminoxanes for use herein include polymeric or oligomeric aluminoxanes, especially methylalumoxane(MAO), isobutylaluminoxane, triisobutyl aluminum modified methylalumoxane, isopropyl alumoxane or diisobutylalumoxane. A preferred aluminoxane is methylalumoxane.
Electrophilic boranes suitable for use herein include tri(hydrocarbyl)boron compounds and halogenated derivatives thereof, having from 1 to 10 carbons in each hydrocarbyl or halogenated hydrocarbyl group, especially tris(fluoroaryl)boranes, tris(trifiuoromethyl substituted aryl)boranes, and tris(pentafluorophenyl)borane. Preferably, the borane is tris(pentafluorophenyl)borane. Activating cocatalysts and activating techniques have been previously taught with respect to different metal complexes in the following references: EP-A-277,003, U.S. Pat. Nos. 5,153,157, 5,064,802, EP-A-468,651, EP-A-520,732, and WO93/23412, the teachings of which are hereby incorporated by reference.
The aluminoxane and electrophilic borane compound are preferably premixed prior to their addition to the Group 4 metal catalyst. Typical mole ratios of aluminoxanelborane are from 1:1 to 150:1, preferably from 2:1 to 100:1, more preferably from 3:1 to 50:1, and most preferably from 5:1 to 20:1.
The cocatalyst premix composition of aluminoxane/borane is preferably contacted with the Group 4 metal catalyst prior to the polymerization. Typical mole ratios of catalyst/cocatalyst composition are 1:5:1 to 1:150:20 catalyst:aluminoxane:borane, preferably 1:25:2 to 1:125:15 catalyst:aluminoxane:borane and most preferably 1:50:4 to 1:75:8 catalyst:aluminoxane:borane.
The foregoing activating cocatalyst composition can also be used in combination with a tri(hydrocarbyl)aluminum compound having from 1 to 10 carbons in each hydrocarbyl group. These aluminum compounds are usefully employed for their beneficial ability to scavenge impurities such as oxygen, water, and aldehydes from the polymerization mixture. The molar ratio of aluminum compound to metal complex is preferably from 10,000:1 to 1:1, more preferably from 5000:1 to 10:1, most preferably from 200:1 to 25:1.
Preferred aluminum compounds include C2-6 trialkyl aluminum compounds, especially those wherein the alkyl groups are ethyl, propyl, isopropyl, n-butyl, isobutyl, pentyl, neopentyl, or isopentyl, dialkyl(aryloxy)aluminum compounds containing from 1-6 carbons in the alkyl group and from 6 to 18 carbons in the aryl group, (especially 3,5-di(t-butyl)-4-methylphenoxy)diisobutylaluminum.
An especially preferred activating cocatalyst composition comprises the combination of a trialkyl aluminum compound having from 1 to 4 carbons in each alkyl group, tris(pentafluorophenyl)borane and methylaluminoxane in a molar ratio from 5:1:25 to 25:8:75.
The catalyst composition is preferably prepared separately prior to contact with polymerizable monomers. In general, the catalyst composition can be prepared by combining the metal complex and activating cocatalyst composition at a temperature within the range of from 10xc2x0 C. to 100xc2x0 C. The components of the catalyst composition can optionally be combined in an inert solvent such as toluene. The catalysts"" components are sensitive to both moisture and oxygen and should be handled and transferred in an inert atmosphere.
The catalyst composition can also be a neat solution obtained by combining the components in the absence of diluent. When MAO is used as the activating cocatalyst, the catalyst premix will comprise from about 45 to 85 percent solvent due to the solvent present in commercial MAO.
Additionally, the polymerization can be conducted in the presence of a catalyst adjuvant. Catalyst adjuvants such as alkylsilane, substituted alkylsilanes, dialkylsilanes, substituted dialkylsilanes, arylsilanes, diarylsilanes, substituted arylsilanes or substituted diarylsilanes can also be used. Preferred adjuvants include diphenylsilane and phenylsilane.
The polymerization may be conducted in the presence of an inert diluent or solvent or in the absence thereof, that is, in the presence of excess monomer. Examples of suitable diluents or solvents include C6-20 aliphatic, cycloaliphatic, aromatic and halogenated aliphatic or aromatic hydrocarbons, as well as mixtures thereof. Preferred diluents comprise the C6-10 alkanes, toluene and mixtures thereof. A particularly desirable diluent for the polymerization is iso-octane, iso-nonane or blends thereof such as Isopar-E(trademark), available from Exxon Chemical Company. Suitable amounts of solvent are employed to provide a monomer concentration from 5 percent to 100 percent by weight. The polymerization can also be conducted under any suitable conditions such as bulk, slurry, or suspension polymerization conditions.
The polymerization is preferably conducted under solventless conditions such as bulk polymerization conditions or other suitable reaction conditions including solid, powdered reaction conditions. The polymerization can be conducted at temperatures of from 0xc2x0 C. to 160xc2x0 C., preferably from 25xc2x0 C. to 100xc2x0 C., more preferably from 30xc2x0 C. to 80xc2x0 C., for a time sufficient to produce the desired polymer. Typical reaction times are from one minute to 100 hours, preferably from 1 to 10 hours. The optimum reaction time or reactor residence time will vary depending upon the temperature, solvent and other reaction conditions employed. The polymerization can be conducted at subatmospheric pressure as well as super-atmospheric pressure, suitably at a pressure within the range of 1 to 500 psig (6.9 kPa-3,400 kPa). The use of ambient or low pressures, for example, 1-5 psig (6.9-34.5 kPa) is preferred in view of lower capital and equipment costs.
The molar ratio of the monovinylidene aromatic monomer to metal catalyst (in terms of Moles) may range from 100:1 to 1xc3x971010:1, preferably from 1000:1 to 1xc3x97106:1.
As in other similar polymerizations, it is highly desirable that the monomers employed be of sufficiently high purity that catalyst deactivation does not occur. Any suitable technique for monomer purification such as devolatilization at reduced pressures, contacting with molecular sieves or high surface area alumina, deaeration, or a combination thereof may be employed.
Purification of the resulting polymer to remove entrained catalyst and cocatalyst may also be desired by the practitioner. Such contaminants may generally be identified by residues of ash on pyrolysis of the polymer that are attributable to catalyst or cocatalyst metal values. A suitable technique for removing such compounds is by solvent extraction, for example, extraction utilizing hot, high boiling chlorinated solvents, acids or bases such as caustic followed by filtration.